1. Field of the Invention
This invention pertains generally to circuitry for electronically controlled acoustic musical instruments, and more particularly to note actuator feedback circuits within player pianos.
2. Description of Related Art
Player pianos continue to get more sophisticated in the pursuit of optimizing performance reproduction. However, the increased sophistication often leads to increased cost factors which reduce market penetration in certain market segments and limit both market advantage and profits.
One of these cost factors arises from the use of real-time feedback circuitry which assures that each actuator provides a similar response to a given stimulus, thus normalizing actuator outputs “on-the-fly”. In order to provide accurate playback, player piano systems employ real-time feedback for sensing the motion of the key mechanism, (i.e. plunger, piano key or hammer) and applying correction to actuator signals while the actuator is active during a key strike. One such form of feedback comprises a Hall Effect sensor coupled for sensing the velocity of the mechanism in response to actuator activity. The measured velocity is compared with the expected velocity and a correction factor applied during the keystroke to correct for error. The real-time feedback thus assures that the actual output properly tracks the expected output. The feedback is typically applied continuously over the key strike in the case of analog circuitry or periodically during the keystroke in the case of digital circuitry.
It will be recognized that sensing hardware, such as Hall Effect sensors, must be coupled to each of the eighty-eight (88) key mechanisms of the piano. Electronic circuitry is required for registering the data from these sensors and generating output corrections in real-time for all of the eighty-eight (88) keys. At least one feedback circuit function, such as a comparator, is required per piano key and more typically multiple comparators, op-amps, and/or other circuitry are necessary to generate proper real-time feedback. Attempts have been made at reducing or integrating analog elements in the feedback path, while multiprocessing has also been proposed which allows each of multiple processors to generate real-time feedback for a subset of the keys.
FIG. 1 represents the major functional elements and their interactions in a solenoid drive circuit 10 having real-time feedback and a performance recording system 12. A piano key 14 is shown operably coupled to a hammer mechanism 16 which translates the mechanical input on key 14 to a hammer motion for striking string 18. In a player piano system a solenoid 20 is coupled to hammer mechanism 16 for striking string 18 during playback of a note sequence. Solenoid 20 is shown comprising coil 22 and plunger 24. Plunger 24 is driven in response to the level of current passing through coil 22 to create a mechanical input to mechanism 16. It should be recognized that the coil current mentioned is preferably the average current level when the solenoid drive is controlled by a preferred pulse-width modulation (PWM) mechanism in which the duty cycle is modulated. It should also be recognized that other actuator types may be similarly utilized in place of solenoids.
A note stream is shown received by a controller 26, such as a microprocessor or other programmable element, which is configured to generate an output for each of the typically 88 keys for controlling the velocity of the hammers as they strike the strings, such as string 18. Typically the note information stream comprises musical instrument note control information formatted according to the musical instrument data interchange (MIDI) standard. It should be readily recognized that each combination of solenoid 20 and hammer mechanism 16 responds slightly differently to a given driving signal. Real time feedback was introduced into the playback portions of the circuit to correct the outputs so that hammer velocity is adjusted to match expected velocity as generated from the controller.
Real-time feedback corrects the solenoid driving force during a key strike to normalize the output which is in progress. The use of real-time feedback is implemented in a number of different ways within the industry. In this example controller 26 is shown outputting a first signal, with the connection of eighty-seven additional outputs not shown, to a first solenoid driver 28 which sinks drive current through coil 22 of solenoid 20 to drive plunger 24. Driver 28 is depicted as an operational amplifier connected in an inverting amplifier mode with circuits providing real-time feedback to control amplifier gain. A sensor 30 is mechanically coupled for sensing the velocity of mechanism 16 to provide input to a real time feedback stage 32 whose transfer function in the feedback path changes in response to sensed velocity in mechanism 16 during a key strike at the desired note velocity. Feedback stage 32 may comprise circuitry such as operational amplifiers, filters, and threshold comparators. It should be noted that since the feedback to driver circuit 28 is provided during playback of the key (key strike) it compensates in real time to normalize strike velocity. It should also be recognized that digital equivalents to the analog block diagram shown in the figure have been described by player piano manufacturers for providing real-time actuator feedback.
Player pianos are often adapted for recording user performances for later playback as depicted in the figure by assembly 12. In registering performances a sensor 34 is coupled to each key 14 (or alternatively through mechanism 16) for registering key movement. The analog movement signal from sensor 34 is converted with an analog-to-digital converter 36 to a digital velocity signal and processed within a controller 38 along with signals from the typically eighty seven other keys to generate a note stream which can be stored in data store 40 (i.e. hard disk, memory device, etc.) for later selection and playback.
Implementing real-time feedback requires incorporating a number of circuits for each key and is thus costly in terms of both the circuitry needed as well as for the necessary printed circuit board real estate for interconnecting those circuits.
Accordingly, a mechanism for providing accurate control of actuator activation is needed which can be implemented at low cost with minimal circuitry associated with each key of the piano. The present invention fulfills that need and others while overcoming drawbacks with existing techniques.